The article I linked to above suggests that the book was made up for the most part of letters exchanged between JRR and Hilary Tolkien and their wives. The Tolkien Estate would own the copyright of JRRT's letters and either the Estate or the surviving children of Edith's so they have the right to give or refuse permission to publish depending on how the letters are used. It seems likely that letters exchanged between the families would relate more to family matters than to Tolkien's work; perhaps the Estate is simply protecting the privacy of his children and grandchildren.

The other thing I though might be significant in the article is that little comment about the author's theories:

Quote

Neil Holford is a reader of all things Tolkien and a long-standing member of the Tolkien Society. . . . Neil worked in the background on Black & White Ogre Country and was chosen to assist Angela with Wheelbarrows at Dawn for his researching skills and his ability to make Angie prove her theories

Maybe she was placing some interpretation on the letters which the Tolkien Estate doesn't agree with.

But I'm only guessing. Whatever it is, it must be something they take very seriously.